1. Field of the Invention
This invention relates generally to a window glass antenna system and more particularly to an automobile telephone antenna system disposed on an automobile window glass for receiving and transmitting automobile telephone transmissions in the Ultra High Frequency (UHF) band.
2. Description of the Related Art
In Japanese Utility Model Laid-Open Publication No. HEI 5-82113, the assignee of this invention proposes a window glass antenna system as schematically illustrated in FIG. 7 hereof.
The proposed window glass antenna system 50 is designed for use with an automobile telephone of UHF band and comprises a radiation pattern 52 disposed on a surface of an automobile window glass 51 and having a vertical length of about 1/4 wavelength, and a grounding pattern 53 having a vertical length of about 1/4 wavelength and a horizontal length of about 1/4-3/4 wavelength, the grounding pattern 53 being formed of an outer frame pattern and a central pattern to thereby provide a centrally-opened configuration. A feeding point "A" is connected to a central portion of a coaxial cable 54 via a center conductor 54a while a ground point "B" is connected to a peripheral portion of the coaxial cable 54 via an outer conductor 54b. The center conductor and the outer conductor have limited lengths of 5 mm or less.
However, difficulty may be experienced in positioning the prior window glass antenna system 50 at the side edge of the window glass 51, because the grounding pattern 53 has an increased transverse dimension and the radiation pattern 52 is located centrally upwardly of the grounding pattern 53.
In Japanese Patent Laid-Open Publication No. HEI 6-237109, the assignee of the present invention also proposes another window glass antenna system as schematically illustrated in FIG. 8 hereof.
This prior window glass antenna system 60 includes a grounding pattern 65 having a reduced transverse length so that it can be easily mounted to a side edge portion of a window glass 61. The antenna system 60 also includes a radiation pattern 63, a signal retrieval pattern 64 extending from a feeding point "A" disposed at the side edge portion proximate to a side edge of the window glass 61, toward the radiation pattern 63, and a shield pattern 66 extending from a ground point "B" provided at the side edge portion proximate to the side edge of the window glass 61, around the feeding point "A" and on both sides along the entire length of the signal retrieval pattern 64, and having a balance-to-unbalance transformer. The grounding pattern 65 is generally rectangular in shape and connected to an end of the shield pattern 66. The feeding point "A" is connected to a central portion of a coaxial cable 67 via a center conductor 67a while the ground point "B" is connected to a peripheral portion of the coaxial cable 67 via an outer conductor 67b. Designated by reference numeral 62 are defogging heaters connected at both ends by common feeding terminals or bus bars 62b and 62c.
In this window glass antenna system 60, however, because the grounding pattern 65 is reduced in a transverse length to thereby enable easy mounting of the antenna system to the side edge portion of the window glass, it becomes necessary to provide the shield pattern 66 for connecting the ground point "B" and the grounding pattern 65, which causes impedance mismatching to thereby decrease the antenna gain.